Kaizoku to Ningyo

Summary

During the time when the world is full of barren lands and big ships sail out to the vast oceans, a very rare type of sub-human–mermaid–was born. Normally, they have white hair, but only in water does the color change. Among the aristocrats, they are traded for large sums of money. Hisui is a merman with scarlet hair, and he's kept by a lord. However, as he's stolen away by the pirate Ikuta and gains freedom, they start their journey together! (Source: Aerandria)